1. Field of the Invention
This invention relates to solid state image pick-up devices, and is particularly concerned with a video camera including a solid state image sensor for deriving a still image video signal.
2. Description of the Prior Art
It is known to provide a solid state image pick-up device including a solid state image sensor such as a charged coupled device (CCD), which can derive an output video signal during one field period or one frame period, so as to form one complete still image video signal.
If the CCD is of the so-called interline transfer type, the picked-up charge signals due to the irradiation of the incident light on photo-sensitive elements of the CCD are transferred to vertical shift registers continuously during each field or frame period. However, in this type of CCD, a so-called smearing effect or smearing noise can occur because some unwanted charges which are not picked-up or captured in the photo-sensitive regions leak out to the regions of the vertical shift registers. Such smearing is particularly likely to occur when incident light having a long wavelength, such as red light, is irradiated onto the CCD. The effect is to cause a blurred image when the signal is reproduced on a television receiver.
Moreover, dark-current noise can also occur, this being due to thermal excitation of electrons in the silicon substrate of the CCD. The extent of the thermal excitation depends on the thermal condition of the substrate and occurs even if the photo-sensitive regions are shielded from incident light by a shutter.
If the amounts of the dark-current charges accumulate in different photo-sensitive regions differ in successive fields, that is in successive odd and even field periods, then the output levels of the picked-up video signals, which are made up of the combination of the charges due to the incident light and the dark-current charges, will also differ. The dark-current has the effect of adding a direct current component to each video signal. In consequence, if the dark currents are different in odd and even field periods, the overall levels of the video signals will be different and this will cause flicker on a television receiver when the video signals are reproduced. This flicker will hereinafter be called flicker of the first kind.
Attempts have been made in the past to overcome both smearing and dark-current effects, but these have resulted in solid-state image sensors which are extremely complicated in structure and expensive to make because of the highly advanced techniques which are used in manufacturing them.
In the case of a video camera which is to produce a still image video signal for reproduction on a television receiver, the output video signal must be in the general form of a standard television signal such as an NTSC, PAL or SECAM signal. Thus, the output video signal of such a video camera should comprise a frame signal consisting of two field signals which are raster interleaved with each other in the usual way.
The need to have two field signals which are derived sequentially introduces a further problem of flicker when picking up an image of a moving object. This is because of the movement between the times of the two fields, which has the result that the video signals of the individual fields are spatially different from one another. This leads in particular to substantial deterioration in the vertical resolution of the reproduced picture. Hereinafter this will be called flicker of the second kind.
Thus flicker of the first kind causes luminance variations in the reproduced picture on a television receiver and flicker of the second kind causes spatially displaced images on the television receiver. Moreover, the smearing results in colour blurring and this is spatially non-uniform.